This proposal will probe molecular mechanisms underlying motor neuron degeneration arising from abnormal NF gene expression. The proposed studies are based on recent discoveries that NF-induced motor neuron degeneration in transgenic mice is due to expression of untranslated neuropathic elements in the 3'UTR of NF-L mRNA. RNA-mediated neuronal degeneration is believed to be mediated by RNA-binding proteins that bind to and regulate the processing of NF-L mRNA and other neuronal transcripts. The presence of neuropathic RNA elements is believed to bind and alter the compositions of RNA-binding proteins in a manner that impairs their ability to maintain the viability of motor neurons. The proposed studies will utilize primary cultures of motor neurons as an in vitro model to identify and characterize the cis-acting elements and trans-acting factors mediating the neuropathic effects of NF-L mRNA. Neuropathic mutations will be introduced into the 3'UTR of endogenous NF-L genes by homologous recombination and the targeted mutations incorporated into the mouse genome by breeding chimeric mice. In this manner, untranslated neuropathic elements will be expressed in a NF-specific pattern in mice bearing a heterozygous or homozygous mutation. The mutant mice should provide a more reproducible phenotype for probing underlying alterations in RNA-mediated pathology in vivo, for examining the synergistic effects of RNA-mediated phenomena on mSOD-1-mediated motor neuron degeneration and for testing the modulating effects of trans-acting factors on RNA-mediated and mSOD-1 mediated motor neuron degeneration